Bacterial Work: New mutants regulating recA dependent DNA replication: New mutants linked to the recA locus and to the dnaT locus have been isolated. The mutations linked to recA appear to be similar to the tif mutants already described in the literature. The mutation linked to dnaT results in all replication becoming conditional on recA activity. When such a mutant is deprived of recA activity replication stops. In vitro replication is absent from lysates prepared from such cells, but is restored by adding purified recA product. This restored in vitro activity is inhibited by ATP-gamma-S, suggesting that the synapsing activity of recA is involved. Plant Work: Synthesis of DNA by plant nuclei: An in vitro system of DNA replication has been established utilizing isolated plant nuclei. The isolation of nuclei involves preparation of protoplasts at pH 8.0, their isolation with non-ionic detergents and purification of nuclei. These nuclei synthesized low molecular weight DNA and joined these nascent pieces into DNA of intermediate molecular weight. After U.V. irradiation of whole cells, both in vivo and in vitro replication were inhibited to a similar extent. After FudR alignment the number of nuclei synthesizing DNA increased corresponding to the increase of cells in "S" phase. Restriction analysis indicated that more than 50 percent of the in vitro replicating DNA consisted of repeated sequences. However, not all repeated sequences were replicated.